A conventional exhaust gas purifier is known to purify nitrogen oxide (hereinafter, refer to as NOx) in exhaust gas. The exhaust gas purifier includes a urea water supply system, which supplies urea water to the exhaust gas, and a selective reduction catalyst, into which the exhaust gas supplied with the urea water flows. For example, when water in the urea water is excessively evaporated, or when a tank is filled with fluid that does not meet standards, an abnormality occurs in the quality of the urea water. Variation in the quality of the urea water changes the amount of urea water necessary for obtaining desired purification performance in the exhaust gas purifier. On this account, the technique disclosed in Patent Document 1 causes a sensor to detect the concentration of the urea water in the tank, and suitability of the urea water is determined based on the detected value. The driver is warned when an abnormality occurs in the quality of the urea water.